Electroluminescent displays are advantageous by virtue of their low operating voltage with respect to cathode ray tubes, their superior image quality, wide viewing angle and fast response time over liquid crystal displays, and their superior gray scale capability and thinner profile as compared to plasma display panels.
As shown in FIGS. 1 and 2, an electroluminescent display has two intersecting sets of parallel, electrically conductive address lines called rows (ROW 1, ROW 2, etc.) and columns (COL 1, COL 2, etc.) that are disposed on either side of a phosphor film encapsulated between two dielectric films. A pixel is defined as the intersection point between a row and a column. Thus, FIG. 2 is a cross-sectional view through the pixel at the intersection of row ROW 4 and column COL 4, in FIG. 1. Each pixel is illuminated by the application of a voltage across the intersection of the row and column defining the pixel using row and column drivers (not shown) coupled to the rows and columns.
Matrix addressing entails applying a voltage below the threshold voltage to a row while simultaneously applying a modulation voltage of the opposite polarity to each column that bisects that row. The voltages on the row and the columns are summed to give a total voltage in accordance with the illumination desired on respective sub-pixels, thereby generating one line of the image. An alternate scheme is to apply the maximum sub-pixel voltage to the row and apply a modulation voltage of the same polarity to the columns that intersect that row. The magnitude of the modulation voltage is up to the difference between the maximum voltage and the threshold voltage to set the pixel voltages in accordance with the desired image. In either case, once each row is addressed, another row is addressed in a similar manner until all of the rows have been addressed. Rows that are not addressed are left at open circuit. The sequential addressing of all rows constitutes a complete frame. Typically, a new frame is addressed at least about fifty (50) times per second to generate what appears to the human eye as a flicker-free video image.
In order to generate realistic video images with flat panel displays, it is important to provide the required luminosity ratios between gray levels where the driving voltage is regulated to facilitate gray scale control. This is particularly true for electroluminescent displays where gray scale control is exercised through control of the output voltage on the column drivers for the display.
Traditional thin film electroluminescent displays employing thin dielectric layers that sandwich a phosphor film between driving electrodes is not amenable to gray scale control through modulation of the column voltage, due to the very abrupt and non-linear nature of the luminance turn-on as the driving voltage is increased. By way of contrast, electroluminescent displays employing thick, high dielectric, constant dielectric layered pixels have a nearly linear dependence on the luminance above the threshold voltage, and are thus more amenable to gray scale control by voltage modulation. However, even in this case if the gray scale voltage levels are generated by equally spaced voltage levels then the luminance values of the gray levels are not in the correct ratios for video applications.
The gray level information in a video signal is digitally encoded as an 8-bit number or code. These digital gray level codes are used to generate reference voltage levels V.sub.g that facilitate the generation of luminance levels (Lg) for each gray level in accordance with an empirical relationship of the form:Lg=f(Vg)=Anγ  (Equation 1)
where:
A is a constant;
n is the gray level code; and
γ is typically between 2 and 2.5.
An electroluminescent display driver with gray scale capability resembles a digital-to analog (D/A) device with an output buffer. The purpose is to convert an incoming 8-bit gray level code from the video source to an analog output voltage for electroluminescent display driving. There are various types of gray scale drivers employing different methods of performing the necessary digital-to-analog conversion. A preferred type and method uses a linear ramping voltage as a means of performing the D/A conversion. For this type of gray scale driver, the digital gray level code is first converted to a pulse-width through a counter operated by a fixed frequency clock. The time duration of the pulse-width is a representation of, and corresponds to, the digital gray level code. The pulse-width output of the counter in turn controls the turn-on of a capacitor sample-and-hold circuit which operates in conjunction with an externally generated linear voltage ramp to achieve the pulse-width to voltage conversion. Since the voltage ramp has a linear relationship between the output voltage and time, the pulse-width representation of the digital gray level code results in a linear gray level voltage at the driver output. The luminance created for each gray level is thus dependent on the relationship between the voltage applied to a pixel and the pixel luminance, which is dependent on the electro-optical characteristic of the electroluminescent display. This luminance-voltage characteristic is normally different from the ideal characteristic, and therefore Gamma correction is necessary.
The relationship between the voltage applied to a pixel and its luminance is typified by the curve in FIG. 3. To achieve proper color balance for the electroluminescent display, a Gamma correction is made to the linear voltage ramp to achieve the relationship between luminance and a gray level given by Equation 1. For the luminance versus voltage curve of FIG. 3, the linear voltage ramp is replaced by the non-linear voltage ramps shown in FIG. 4. The non-linear voltage ramps can be generated using analogue circuitry such as that taught in co-pending U.S. patent application Publication No. 2004/0090402 to Cheng or by other means as may be known in the art. The non-linear voltage ramps are different for positive and negative row voltages because in the former case the pixel voltage is the difference between the row and column voltages and in the latter case the pixel voltage is the sum of the row and column voltages. The luminance begins to rise above the threshold voltage in a non-linear fashion for the first few volts above the threshold voltage, and then rises in an approximate linear fashion before saturating at a fixed luminance. The portion of the curve used for electroluminescent display operation is the initially rising portion and the linear portion. The effects of differential loading of the driver outputs complicate the relationship. To negate the effect of variable loading and to improve the energy efficiency of the electroluminescent display, a driver employing a sinusoidal drive voltage with a resonant energy recovery feature is typically employed. Such a driver is disclosed in U.S. Pat. No. 6,448,950 to Cheng and U.S. patent application Publication No. 2003/0117421 to Cheng, the contents of which are incorporated herein by reference. U.S. patent application Publication No. 2004/0090402 to Cheng teaches a method and apparatus to realize the necessary Gamma correction of an electroluminescent display panel conveniently at the D/A conversion stage by replacing the normal linear voltage ramp with a special ‘double-inverted-S’ non-linear voltage ramp. The use of this non-linear voltage ramp enables adjustment of the voltages for the gray levels to generate a gray scale response similar to that described by the empirical relationship given by Equation 1.
As described in U.S. Pat. No. 6,448,950 to Cheng, a major portion of the power consumed by passively addressed electroluminescent displays is fed through the column drivers due to a parasitic capacitive coupling between the columns and the non-addressed rows. This patent teaches a means to reduce this power consumption by providing a sinusoidal driving waveform to minimize peak current and to recover a major portion of the energy through a resonant energy recovery circuit. Co-pending U.S. Provisional Patent Application No. 60/646,326 filed on Feb. 23, 2005 teaches a means to increase further the energy efficiency by ensuring that as much of the energy from the electroluminescent display panel is recovered by the energy recovery circuit and not dissipated in parallel parasitic current loops through ground and through the supply voltage lines for the drivers. Although, these measures provide for energy recovery, they do not reduce the current flow through the drivers to zero. As will be appreciated, improvements in electroluminescent display energy efficiency and cost reductions in the column drivers may also be realized if the current flowing from the output of the column drivers can be reduced.
Other techniques for driving electroluminescent displays have been considered. For example, U.S. Pat. No. 6,636,206 to Yatabe discloses a system and method of driving a display device so as to display a gray scale image without causing a significant increase in power consumption. Pixels disposed at locations corresponding to respective intersections of a plurality of scanning lines extending along rows and a plurality of data lines extending along columns are driven. A single scanning line is selected during one horizontal scanning period and a selection voltage is applied to the scanning line for one half of the scanning period. Another adjacent scanning line is selected during the next horizontal scanning period and the selection voltage is applied to the scanning line for the other half of the scanning period. At the same time, a turn-on and turn-off voltage is applied to a pixel at a location corresponding to the selected scanning line such that the turn-on voltage is applied for a length corresponding to a gray level in the period during which the selection voltage is applied. The turn-off voltage is applied during the remaining period.
U.S. Pat. No. 5,315,311 to Honkala discloses a method and apparatus for reducing power consumption in an AC-excited electroluminescent display. Each row of the display matrix is alternatively driven by positive and negative row drive pulses. The magnitudes of successive row drive pulses are different. Each column of the display matrix is driven individually by modulation voltage pulses synchronized to the row addressing sequence. The modulation voltage pulses have a maximum amplitude and an “on”-state polarity equal to that of the larger-magnitude row drive pulse.
U.S. Pat. No. 6,803,890 to Velayudehan et al. discloses a system and method for addressing and achieving gray scale in an electroluminescent display using a waveform having at least one positive ramped modulating pulse and zero or more non-ramped modulating pulses. The pulses are applied to the electroluminescent display successively to form a scan pulse that is applied across an electrode row and electrode column.
Although various techniques for driving electroluminescent displays exist, improvements are continually being sought. It is therefore an object of the present invention to provide a novel electroluminescent display using bipolar column drivers.